The present invention relates generally to waveguide lasers and, more particularly, to an apparatus and method for applying modulating signals to piezoelectric transducers at both ends of a waveguide laser cavity to provide increased cavity length modulation.
In a typical gas waveguide laser system, it is known to frequency modulate the energy output from the laser by modulating the length of the laser cavity. The laser output frequency can be expressed as f=c/2L, where c is the velocity of light and L is the length of the optical cavity.
In current applications, the cavity length is typically controlled by mounting the mirrors at both ends of the cavity on transducers. The transducers used for this application are crystals which expand or contract a precise distance with an applied voltage. This property is known as the piezoelectric effect, and the crystals are referred to as piezoelectric transducers or PZT's.
In these applications, one of the PZT's is used to frequency stabilize the laser, or in other words, to keep the cavity length constant. This PZT is controlled by servo electronics and protects the cavity length against changes due to thermal or mechanical disturbances. Because this PZT "tunes" the cavity length, it is referred to as the tuning PZT. The tuning PZT has a low bandwidth and a relatively high sensitivity, on the order of nanometers per volt.
The other PZT has a high bandwidth and is used to modulate the cavity length. Since this PZT induces an optical frequency modulation, it is referred to as the FM PZT. The FM PZT throws on the order of hundreds of picometers per volt, i.e., roughly one-tenth the response of the tuning PZT. This frequency modulation is superimposed onto the tuning of the laser cavity. The result is a locked cavity length and therefore a locked laser frequency with a superimposed frequency modulation controlled by the shape and amplitude of the drive waveform input to the FM PZT.
The amount of frequency deviation that can be achieved is limited by the performance of the FM PZT. Present systems are driving the PZT to its maximum voltage and as a result the PZT is behaving in a non-linear manner.